Di�erences in semantic category priming in the left and rightcerebral hemispheres under automatic and controlled processing

conditions

Marjorie Collins*

School of Psychology, Murdoch University, Murdoch, Perth, WA 6150, Australia

Received 28 January 1998; accepted 9 November 1998

Abstract

The contribution of each cerebral hemisphere to the generation of semantic category meanings at automatic and strategiclevels of processing was investigated in a priming experiment where prime and target words were independently projected to the

left or right visual ®elds (LVF or RVF). Non-associated category exemplars were employed as related pairs in a lexical decisiontask and presented in two experimental conditions. The ®rst condition was designed to elicit automatic processing, so relatedpairs comprised 20% of the positive set, stimulus pairs were temporally separated by a stimulus onset asynchrony (SOA) of 250ms, and there was no allusion to the presence of related pairs in the instructions to subjects. The second condition, designed to

invoke controlled processing, incorporated a relatedness proportion of 50%, stimulus pairs separated by an SOA of 750 ms, andinstructions which informed subjects of the presence and use of category exemplar pairs in the stimulus set. In the ®rstcondition, a prime directed to either visual ®eld facilitated responses to categorically related targets subsequently projected to the

RVF, while in the second condition a prime directed to either visual ®eld facilitated responses to related targets projected to theLVF. The facilitation e�ects obtained in both conditions appeared to re¯ect automatic processes, while strategic processes wereinvoked in the left, but not the right hemisphere in the second condition. The results suggest that both hemispheres have

automatic access to semantic category meanings, although the timecourse of activation of semantic category meanings is slowerin the right hemisphere than in the left. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Cerebral hemispheres; Priming; Semantic category; Automatic processing; Controlled processing; Semantic memory

1. Introduction

Recent research on the contribution of each cerebralhemisphere to linguistic processing has focused uponhemispheric di�erences in the use of two fundamen-tally di�erent ways of organising information aboutthe interrelationships between real-world entities,namely, associative and semantic category relation-ships. This has been in response to Neely's suggestion[25] that a clear distinction should be made betweenthe representation of these two types of relationship,since priming e�ects may be highly dependent upon

the precise status of the relationship between two lin-guistic entities. The need for such a distinction hasbeen borne out by studies of priming in the cerebralhemispheres which have found di�erences in the prim-ing of semantic category and associative relationships[e.g. 7,13,14]. However, despite the consensus that cat-egorical and associative relationships may be activateddi�erently in each hemisphere, there are discrepanciesin the ®ndings from studies investigating this issue.

For instance, Chiarello, Burgess, Richards andPollock [6] examined hemispheric priming for wordpairs which were associated, non-associated membersof the same semantic category, or category memberswhich were also associated. For lexical decisions withpairs presented in the left or right visual ®eld, no prim-ing was evident in either hemisphere for prime-target

Neuropsychologia 37 (1999) 1071±1085

0028-3932/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0028-3932(98 )00156 -0

* Tel.: +61-8-9360-2186; fax: +61-8-9360-6492.

E-mail address: collins@central.murdoch.edu.au (M. Collins)

pairs related by association only. Priming wasobserved, to an equal degree in each hemisphere, forassociates that happened to be members of the samesemantic category. However, when word pairs weremembers of the same semantic category, but not as-sociated with one another, priming was present only inthe right hemisphere. These ®ndings were con®rmed ina subsequent study by Chiarello and Richards [7]which investigated the possibility that exemplar domi-nance is instrumental in in¯uencing the direction andmagnitude of hemispheric di�erences in automaticpriming of semantic category members. They found noe�ects for exemplar dominance, but priming for non-associated category pairs was once again restricted tothe right hemisphere. The ®ndings from these two stu-dies raise the possibility that the right hemisphere pre-dominates in access to semantic category relationships.

However, these results con¯ict with those of otherpriming studies investigating the same issue. (Collins)Abernethy1 and Coney [12] found priming restricted tothe left hemisphere for word pairs from the samesemantic category. This supports the notion that it isthe left hemisphere which specialises in semantic cat-egory relationships, which is in direct contrast to the®ndings of Chiarello and her colleagues. In an attemptto resolve these discrepancies, (Collins) Abernethy andConey [14] looked more carefully at hemispheric prim-ing for semantic category relationships. We noted thatthe temporal interval separating the presentation ofprime and target (i.e. SOA) was considerably longer inthe studies of Chiarello and colleagues [6, 7] than inour earlier study [12]. In view of our previous ®ndingthat lexical representations are activated more slowlyin the right hemisphere than in the left [13], we investi-gated whether this factor would account for the discre-pancies in priming research. The ®ndings werecompletely in accord with those of our previous study[12]: semantic category priming was present within theleft, but not the right hemisphere. By using nonasso-ciated semantic category pairs, and minimizing thenumber of pairs drawn from each semantic category,we also eliminated the factors Chiarello and Richards[7] had used as explanations for the discrepancies inthis area. Evidently, the discrepancies are attributableto some other factor/s.

It is conceivable that these discrepancies arise fromthe measurement of di�erent underlying cognitive pro-cesses, where Chiarello and colleagues [6,7] havemeasured automatic processes, while (Collins)Abernethy and Coney [14] have measured controlledprocesses. Automatic processing refers to that com-ponent of cognitive information processing where astimulus (such as a word) automatically activates its

internal mental representation, and this activationspreads to related representations in memory [10]. Thisprocess is fast acting, occurs without intention or con-scious awareness and is unlimited in capacity [26,27].Priming e�ects are presumed to re¯ect automatic pro-cesses when the temporal interval between stimuluspairs is less than 400 ms, primes are pattern masked,or there is a small proportion of related pairs in thestimulus set [24,25]. Under these conditions, automaticprocesses facilitate responses to target words precededby semantically related primes, but do not in¯uence re-sponses to unrelated targets. Controlled processing incontrast, is slow acting, can only operate with inten-tion and is limited in capacity [26,27]. It can be elicitedwhen conditions encourage the processing of a primeas a predictor of a subsequent target, and so is associ-ated with tasks employing clearly perceptible primes, atemporal interval between stimulus pairs exceeding 500ms, or a high proportion of related pairs in the stimu-lus set [25]. Under such conditions, expectancies aboutwords are used strategically to direct attention to alimited domain of related meanings in memory. Whileboth automatic and controlled processes facilitate re-sponses to targets preceded by related primes, it isonly under controlled processing conditions that re-sponses to unattended stimuli, such as unrelated tar-gets, are inhibited relative to a neutral condition[26,27]. This inhibition can result from two di�erentstrategic processes: expectancies and retrospectivematching. The former arises when expectancies aboutlikely targets are violated as unrelated targets appear,which results in increased processing time and slowerresponses to unrelated targets [24]. Retrospectivematching occurs in the lexical decision task when therelatedness between a prime and target is used strategi-cally after lexical access to decide whether a target is aword or nonword. If a relationship between wordpairs is identi®ed, as is the case in related word pairconditions, a rapid `word' decision can be made.However, when there is no relationship between wordpairs, there is an initial tendency to decide that the tar-get is a `nonword', which stalls the correct `word' re-sponse and reduces accuracy for unrelated word pairsrelative to related pairs [25].

Studies ®nding di�erent pro®les of semantic cat-egory priming in each hemisphere have also di�ered intheir manipulation of the task variables associatedwith automatic and controlled processing. Chiarelloand colleagues have found semantic category primingrestricted to the right hemisphere with a smaller pro-portion of related pairs in the stimulus set as well aslonger intervals between the onset of prime and targetpairs (called SOA) [6,7] than our studies which havefound left hemisphere category priming with a higherrelatedness proportion and shorter SOAs [12,14].Although SOA or relatedness proportion can be1 I have recently changed my surname from Abernethy to Collins.

M. Collins / Neuropsychologia 37 (1999) 1071±10851072

manipulated to invoke automatic or controlled proces-sing [25], it is possible that one of these variables is themajor determinant of the level of processing elicited inthe lexical decision task when stimuli are lateralized.

The proportion of related pairs in the stimulus set isa likely candidate. Postlexical processing has beenfound to contribute to priming e�ects in the lexical de-cision task when a high proportion of stimulus pairsare related, while priming with a low relatedness pro-portion re¯ects the automatic spread of activation [28].Neely [25] has also suggested that retrospective match-ing is likely to occur when the task includes a highproportion of related pairs in conjunction with a highnonword ratio. Under such circumstances, expectationsabout likely targets are a�ected, which biases responsesto unrelated word and nonword targets. Perhaps ourinclusion of a relatively high proportion of categori-cally related pairs (50% of the positive set), combinedwith a high nonword ratio of 66.6%, jointly providedconditions suitable for postlexical processes to predo-minate in generating priming e�ects in our recentstudy [14]. If so, the priming we observed may re¯ectthe in¯uence of postlexical processing in the left hemi-sphere rather than the automatic activation of categ-orical representations therein, despite our use of SOAsunder 500ms.2

This idea gains credence when considering researchwhich suggests that the left hemisphere plays a greaterrole than the right hemisphere at the postlexicalsemantic integration stage, when the meanings ofprimes and targets are compared before selecting a re-sponse. For instance, Chiarello [5] found greatersemantic priming in the left hemisphere under con-trolled processing conditions, and in the right hemi-sphere under automatic processing conditions. Sheconcluded that the left hemisphere plays a more im-portant role at the stage of postlexical semantic inte-gration, while spreading activation is more potent inthe right hemisphere. The primary role of the lefthemisphere in postlexical semantic integration was sub-sequently con®rmed in a study where controlled pro-cessing was evoked in the left, but not the righthemisphere [9]. It is not possible to make a direct com-parison of these two studies with our recent research[14] as the ®rst study did not control the degree of as-sociation between category pairs, and the second usedassociates as stimuli. Even so, it is interesting that inboth of these studies, controlled processing was elicited

in the left hemisphere when 60% of the positive setwas comprised of related pairs, but not when only20% of the positive set was related. One cannot ignorethe fact that these two studies evoked controlled pro-cesses in the left hemisphere with a relatedness pro-portion only 10% higher than the proportion used inour study [14]. This raises the possibility that the prim-ing in our study also re¯ected controlled processes. Ifso, semantic category priming in the left hemispheremay arise from controlled processes, while categorypriming in the right hemisphere re¯ects automatic pro-cesses.

Nevertheless, a recent study by Koivisto [20] indi-cates that SOA may also be an important factor indetermining the pro®le of category priming in eachhemisphere. In an attempt to resolve the discrepanciesbetween the ®ndings in this area, he employed a smallproportion of related pairs in the stimulus set andcompared the magnitude of semantic category primingin each hemisphere for prime-target pairs separated bySOAs ranging from 165 to 750 ms. When these nonas-sociated category pairs were separated by a temporalinterval of 165 ms, he found priming restricted to theleft hemisphere, whereas at the longest interval primingwas present only in the right hemisphere. Moreover,category priming decreased over time in the left hemi-sphere, while the onset of priming occurred much laterin the right hemisphere than in the left. Although therewas no interaction between visual ®eld and the magni-tude of priming at two intermediate SOAs of 250 and500 ms, there was a trend towards category priming inthe right, but not the left hemisphere at the 500 msinterval. These ®ndings indicate that the discrepanciesin priming obtained by (Collins) Abernethy and Coney[12,14] and Chiarello and colleagues [6,7] may be atleast partially attributable to di�erences in the choiceof SOA, where semantic category priming is restrictedto the left hemisphere at short SOAs and to the righthemisphere at long SOAs. Nonetheless, as Koivistoacknowledges, it is possible that the priming e�ects heobtained at the various SOAs re¯ect di�erent levels ofprocessing. It remains to be seen how level of proces-sing is involved in access to semantic category rep-resentations in each hemisphere.

Hence, the current experiment will examine semanticcategory priming in each hemisphere under conditionsspeci®cally designed to elicit either automatic or con-trolled processes. This will allow further clari®cationof present con¯icting ®ndings [14 cf. 6,7] by demon-strating whether the right hemisphere contributes tothe activation of semantic category aspects of wordmeanings at an automatic level of processing while theleft hemisphere primarily invokes conscious processesto generate category information. Three classicmethods of invoking automatic and controlled proces-sing will be used, including manipulation of the tem-

2 This is unlikely to be the case in our earlier study [12] as there

were no signi®cant di�erences between responses to targets preceded

by neutral or unrelated primes, even though the stimulus set included

a relatively high relatedness proportion of 33.3% and a nonword

ratio of 60%. However, the priming e�ects in that study cannot be

unambiguously interpreted, because we failed to control for the

degree of association between semantic category pairs.

M. Collins / Neuropsychologia 37 (1999) 1071±1085 1073

poral interval between stimulus pairs (i.e. SOA), theproportion of related pairs in the stimulus set, andinstructions to subjects [25]. The automatic processingcondition will have a low proportion of related wordpairs, a short SOA, and instructions to subjects whichare devoid of any allusion to related pairs in the stimu-lus set. The controlled processing condition willinclude a higher proportion of related word pairs, ahigher nonword ratio, a long SOA, and task instruc-tions which speci®cally draw attention to the presenceand use of category exemplar pairs in the stimulus set.The resultant pro®le of facilitation and inhibitione�ects will be used to gauge the level of processing eli-cited under these conditions. Facilitation is understoodto re¯ect both automatic and controlled processes,while inhibition occurs only as a consequence of con-trolled processes [24,25]. Following Neely [25], facili-tation is de®ned here as superior performance fortargets following related primes relative to perform-ance for targets following a neutral prime, while inhi-bition is de®ned as superior performance for targetspreceded by a neutral prime relative to targets pre-ceded by unrelated primes (refer to 14 for a discussionof the choice of baseline in hemispheric research).

2. Method

2.1. Subjects

Thirty-eight undergraduate psychology studentsacted as subjects. Data for 9 of these subjects were dis-carded as 6 had error rates exceeding 30% in eitherthe word or nonword condition, and 3 moved theireyes from ®xation on several experimental trials.Fourteen female and 15 male subjects remained, theirmean age being 25.65 years. All had normal or cor-rected-to-normal vision and English was their ®rstlanguage. All were predominantly right handedaccording to Bryden's hand preference questionnaire[3]. The ®nal sample had a mean handedness quotientof+0.8 (SD 0.24), where on a scale from ÿ1.00to+1.00, the maximum possible score for right hand-edness is+1.00.

2.2. Apparatus

Subjects were tested in a well-lit cubicle room on aCommodore AMIGA 1000 microcomputer systemwith a 1084 S monitor employing a display resolutionof 640 � 200 pixels. Stimuli were presented on themonitor screen in white upper-case letters against adark background. Screen intensity was set at a lowlevel to minimize phosphor persistence. Individualletters were presented in a custom-made sans seriffont, and did not exceed the dimensions of 6 � 14 mm.

Subjects viewed the monitor from a distance of 45 cm.An adjustable chin rest was used to stabilize each sub-ject's head in the correct central position and distancefrom the screen, such that all stimuli were presentedwith their innermost boundary, whether to the left orright of centre, exactly 2.5 degrees of visual angle fromthe central ®xation. Reaction time was measured to aresolution of 1 ms via a centrally positioned touch-sen-sitive panel connected to the user port of the micro-computer. Eye movements were monitored by aSANYO video camera connected to a video monitorwhich provided a magni®ed view of the subject's eyes.EMH12 High Performance earmu�s were used tominimize any noise interference.

2.3. Design

Reaction time (RT) was the principal dependentvariable, with errors also recorded and analyzed. Alexical decision task was used, which required subjectsto discriminate words from nonwords using a GO±NOGO response procedure. Four experimental vari-ables were manipulated: processing condition, testingsession, visual ®eld of presentation, and type of stimu-lus pair. Work in my laboratory indicates that thereare individual di�erences in the magnitude of primingin each hemisphere [11,14], so repeated measures weretaken on all variables. For the ®rst experimental vari-able, three classic methods were used to evoke eitherautomatic or controlled processing. To elicit automaticprocessing, related word pairs comprised 20% of thepositive set (and 10% of the total stimulus set), stimu-lus pairs were separated by a short SOA of 250 ms,and there was no allusion to the presence of relatedpairs in the instructions to subjects. In the controlledprocessing condition, the proportion of related wordpairs was increased to 50% of the positive set (and25% of the total stimulus set), pairs were separated bya lengthy SOA of 750 ms, and instructions to subjectshighlighted the presence and use of category exemplarpairs in the stimulus set. To prevent contamination ofresponses in the automatic processing condition byknowledge gained whilst performing the controlledprocessing task, the former condition was always com-pleted ®rst. However, under such circumstances, prac-tice may confound the priming e�ects in the controlledprocessing condition relative to the automatic con-dition. Therefore, the experiment was intentionallydesigned to gauge whether practice interacted with theexperimental variables of interest here. To this end,testing session was included as the second experimentalvariable, where testing for each processing conditionwas undertaken in two separate sessions. A minimumof 30 min separated the two automatic processing ses-sions, 1a and 1b, as well as the two controlled proces-sing sessions, 2a and 2b. At least 2 hours separated

M. Collins / Neuropsychologia 37 (1999) 1071±10851074

sessions 1b and 2a, although most subjects opted toseparate them by a day or more. Practice e�ects willbe identi®able by a reduction in RT over the course ofthe four testing sessions. If session does not interactwith the other three variables, then a clear interpret-ation of priming e�ects in the automatic and con-trolled processing conditions can be made.

The third experimental variable was visual ®eld ofpresentation, which comprised four levels: (i) primeand target to the RVF (i.e. RVF±RVF); (ii) prime tothe LVF and target to the RVF (LVF±RVF); (iii)prime and target to the left visual ®eld (LVF±LVF);(iv) prime to the RVF and target to the LVF (RVF±LVF). The fourth experimental variable was type ofstimulus pair, where a total of 1440 prime-target pairswere presented to subjects in the four experimental ses-sions. These 1440 pairs were divided into four stimuluslists, each comprised of related, neutral and unrelatedword pairs, as well as word±nonword and neutral±nonword pairs. These lists were carefully matched withone another on all relevant variables (see detailsbelow). One list was presented per experimental ses-sion. Lists A and B were presented, in counterbalancedorder, in session 1a or 1b. Lists C and D were pre-sented, also in counterbalanced order, in session 2a or2b. The salient di�erence between these two sets oflists was the proportion of related pairs, where lists Cand D included a higher proportion of such pairs.Related pairs in each list were nonassociated exemplarsdrawn from the same semantic category in publishedsemantic category norms [2,30]. The number of exem-plars drawn from a given semantic category was mini-mized, as outlined below. In each testing session,stimulus pairs were presented in only one visual ®eldcondition. Selection of visual ®eld for each pair wasrandomly determined, as was order of presentation ofword pairs in each condition. Thus, each subject wasexposed to a unique distribution of pairs in each visual®eld, and a di�erent sequence of prime-target con-ditions within and across testing sessions.

2.4. Stimulus Materials

Four stimulus lists were presented. Lists A and Bwere presented in the automatic processing condition,while lists C and D were presented in the controlledprocessing condition. To ensure that the related exem-plar pairs were not associated, 114 undergraduate psy-chology students were asked to generate threeassociates to each of 186 exemplars initially selected aspotential candidates for the related set. None of thepairs included in the related sets of the four lists wereselected as associates by more than one of these sub-jects. All pairs were phonemically dissimilar, and nonebegan with the same phoneme. Orthographic similaritywas minimized, and none of the pairs had the same

®rst letter. All words were concrete, imageable nounsor adjectives. All stimuli were between 3 and 8 lettersin length, with 87% between 3 and 6 letters.

List A was comprised of 400 stimulus pairs, 40 ofwhich were related-word pairs (e.g. RUBY±OPAL;SHEEP±PIG). Each of these exemplar pairs wasdrawn from a di�erent semantic category. The 80 unre-lated word-pairs were generated by re-pairing each ofthe pairs in the related word condition, and supple-menting them with 40 extra pairs. These extra wordswere matched with words in the related-word con-dition on word frequency, grammatical category andlength. None of the unrelated pairs were orthographi-cally, phonemically, categorically or associatively re-lated (e.g. SHEEP±OPAL). The 80 neutral pairs wereformed by pairing the prime `BLANK' with each ofthe 80 targets in the unrelated set (e.g. BLANK±OPAL). The 120 word±nonword pairs were comprisedof 80 additional word primes selected to match theunrelated primes on word frequency, grammatical cat-egory, and length. These primes were then paired withan orthographically legal and pronounceable nonwordtarget. Nonwords were derived by unsystematicallychanging between 1 and 3 letters of the 80 targets inthe unrelated set (e.g. SATAN±OPIZ; REEF±PUC;TUNNEL±CLUFT). To complete the word±nonwordset, and equate the number of repetitions of these stim-uli with those in the positive set, 40 of these additionalword primes were then re-paired with a nonword fromthe same set (e.g. SATAN±PUC; TUNNEL±OPIZ).Lastly, to form 80 neutral control pairs which mirroredthe neutral condition in the positive set, each of thenonword targets was also paired with the word`BLANK' (e.g. BLANK±CLUFT). Mean frequency ofword targets was 31.4 wpm, while primes in the posi-tive and negative sets were 30.89 wpm and 28.69 wpm,respectively [21].

To ameliorate any confounding e�ects resultingfrom repetition of semantic categories [7] or stimuluspairs [17], a unique set of stimuli were selected for listB. Stimuli for this list were selected according to thesame criterion used for list A, and were also matchedwith list A on word frequency, grammatical category,length, and number of repetitions. Mean frequency ofprimes in the positive set of list B was 33.63 wpm,while word targets were 29.25 wpm and primes in thenegative set were 31.1 wpm [21]. For the related-wordpair condition, one new exemplar pair was drawnfrom each of the 40 semantic categories sampled in listA. For example, the pair `RUBY±OPAL' was drawnfrom the semantic category `a precious stone' in list A,so `PEARL±JADE' were also drawn from this cat-egory and used as a related pair in list B.Consequently, for the automatic processing condition,each semantic category had two exemplar pairs drawnfrom it, with one pair included in list A, and the other

M. Collins / Neuropsychologia 37 (1999) 1071±1085 1075

in list B. Related pairs in lists A and B were presentedonce only.

The remaining two stimulus lists were designed toelicit controlled processing. They were presented intesting sessions 2a and 2b. Stimuli in these listsmatched the two lists in the automatic processing con-dition on word frequency, grammatical category andlength. The relatedness proportion in lists C and Dwas increased to 50%. Half of the related pairs in listC were the related pairs previously used in list A,while list D included half of the related pairs from listB. The remaining related pairs in these two lists weredrawn from new semantic categories, with two exem-plar pairs drawn from each category. One of these newexemplar pairs was placed into list C, and the otherinto list D. For example, list C drew the pair `SOFA±RUG' from the new semantic category `an article offurniture' so `TABLE±DESK' was also drawn fromthis category and used as a related pair in list D.Consequently in the controlled processing condition,two exemplar pairs were drawn from a given semanticcategory, with one pair included in List C, and theother in List D. Furthermore, half of the exemplarpairs used in this condition were repetitions of the re-lated pairs used in the automatic processing condition.

List C was comprised of 320 stimulus pairs, 80 ofwhich were related-word pairs. Forty of these were therelated pairs from list A (e.g. RUBY±OPAL) and 40were new exemplar pairs selected from 40 additionalsemantic categories according to the criterion used forlists A and B (e.g. SOFA±RUG). The 40 unrelatedword-pairs were generated in two stages, to ensure thatthe repetitions of stimuli were equated with the relatedset. First, half of the related pairs from list A wereselected, as were half of the new related pairs. Then,each of these primes and targets were re-paired, whilstensuring that none were orthographically, phonemi-cally, categorically or associatively related (e.g.SHEEP±OPAL; SCALPEL±RUG). Forty neutral pairswere formed by pairing the prime `BLANK' with eachof the 40 targets in the unrelated set (e.g. BLANK±RUG). The 120 word±nonword pairs were comprised ofthe 80 pairs from the negative set of list A, althougheach word was re-paired with a di�erent non-wordfrom this set (e.g. SATAN±CLUFT; REEF±OPIZ).To complete this set, an additional 40 word primeswere selected to match the 40 new primes that hadbeen added to the related set of this list, while theremaining 40 nonwords were generated by unsystema-tically changing between 1 and 3 letters of the 40 newword targets added to the related set of this list (e.g.GYM±PEPO). Forty neutral control pairs were gener-ated by pairing each of the 40 new nonwords with`BLANK' (e.g. BLANK±PEPO). Mean frequency ofprimes in the positive and negative sets were 43.64wpm and 41.15 wpm respectively, while word targets

were 33.94 wpm [21]. List D mirrored list C, and wasmatched on the number of stimuli in each condition,word frequency, grammatical category, length andnumber of repetitions. However, where list C includedstimuli from list A, list D included stimuli from thecorresponding conditions of list B. Mean frequency ofprimes in the positive set was 38.29 wpm, while primesin the negative set were 38.65 wpm and word targetswere 32.11 wpm [21].

2.5. Procedure

Prior to the ®rst testing session, subjects were giveninstructions outlining their task as one of distinguish-ing target words from nonwords. No allusion wasmade to the presence of related pairs in the stimulusset, and subjects were advised that they could chooseto ignore the prime. The necessity of maintaining ®x-ation on the central dot during presentation of allstimuli was emphasized. Subjects were then given 40practice trials with the same structure as the exper-imental trials. No stimuli from the experimental setwere used. If the video monitor revealed any deviationof eyes from ®xation during the practice trials, subjectswere reminded of the importance of maintaining ®x-ation at all times. Before the controlled processingtask, subjects were given further instructions directingtheir attention to the presence of categorically relatedpairs in the stimulus set, and were encouraged to thinkof related exemplars after the appearance of eachprime.

Each trial began with a central ®xation cross whichremained on for 450 ms. The cross was then replacedby a central dot. After 150 ms, a prime word was dis-played one line higher than the central dot, and in theLVF or RVF for 150 ms. Either 100 ms or 600 msafter the prime disappeared, the target was presentedto the LVF or RVF for 150 ms. The shorter intervalwas employed in the automatic condition, and thelonger interval in the controlled processing condition.The target appeared directly below the display locationof the prime to minimize masking e�ects, and the cen-tral ®xation dot endured until the target was erasedfrom the screen. The entire screen remained blank for1500 ms, during which the subject signalled a response.Randomization of trials ensured that subjects wereunable to predict the visual ®eld in which either theprime or target would appear.

Subjects responded in accordance with a GO±NOGO procedure. When the target was a word, theywere required to respond by simultaneously depressingtwo centrally positioned touch-sensitive panels withboth index ®ngers, the faster of the two responsesbeing taken as RT for that trial. When the target wasnot a word, they were required to withhold their re-sponse. Subjects were permitted 1500 ms after erasure

M. Collins / Neuropsychologia 37 (1999) 1071±10851076

of the target to respond. Failure to respond within1500 ms was treated as a NOGO response. Followingtrials in which an incorrect response was made, thescreen ¯ashed blue momentarily. Subjects were per-mitted to rest after each block of trials, and were givenfeedback on their accuracy and overall speed for thepreceding block/s. They were also encouraged to main-tain an error rate of less than 5% per block. Therewere 10 blocks of trials in each session, with 40 trialsper block in sessions 1a and 1a, and 32 trials per blockin sessions 2a and 2b.

3. Results

3.1. RT Analyses

Statistical analyses were carried out on mean correctreaction time (RT) for the positive set. To assesswhether practice confounded the priming e�ects in thecontrolled processing condition relative to the auto-matic condition, a preliminary four-way analysis ofvariance (2 � 2 � 3 � 4) was computed on testing ses-sion, processing condition, type of stimulus pair andvisual ®eld of presentation. There was a main e�ectfor session (F(1,28)=17.96; P = 0.001) where RTswere faster in the second testing session of both theautomatic and controlled processing conditions. Thisindicates that there was a general practice e�ect, asexpected. In the automatic condition, RTs were 17 msfaster in session 1b than session 1a, at 542 ms and 559ms, respectively. In the controlled condition, responsesin session 2b were 26ms faster than responses in ses-sion 2a, with mean RTs of 542ms and 568ms. It isinteresting to note that RTs in sessions 1b and 2b wereprecisely equivalent. Session did not interact with anyof the other variables, which indicates that practicee�ects were consistent across all conditions.Consequently, interpretation of the RT data can pro-ceed with the assurance that practice in¯uenced re-sponses equally in each of the remaining experimentalconditions, and did not interact with the priminge�ects in either hemisphere under either processingcondition. Thus, further statistical analyses collapsedacross this variable, and reference will henceforth bemade solely to sessions 1 and 2.

A three-way ANOVA (2 � 3 � 4) was then com-puted on processing condition, type of stimulus pairand visual ®eld. There were two main e�ects. Themain e�ect for visual ®eld (F(3,84)=16.01; P < 0.001)re¯ected the faster responses to RVF targets (Figs. 1,2). This is consistent with previous ®ndings of a lefthemisphere advantage in lexical decision [e.g. 1,14,15].The main e�ect for stimulus pair (F(2,56)=23.83;P < 0.001) re¯ected an overall facilitation when tar-gets were preceded by related primes (at 540 ms)

relative to neutral and unrelated primes, which hadsimilar mean RTs (559 ms and 560 ms, respectively).All two-way interactions between these three variableswere signi®cant (processing condition and visual ®eld:

Fig. 2. Mean correct RT for related, neutral and unrelated word

pairs presented in the left and right visual ®elds under controlled

processing conditions.

Fig. 1. Mean correct RT for related, neutral and unrelated word

pairs presented in the left and right visual ®elds under automatic

processing conditions.

M. Collins / Neuropsychologia 37 (1999) 1071±1085 1077

F(3,84)=9.34; P < 0.001; processing condition andstimulus pair: F(2,56)=5.7; P = 0.006; visual ®eld andstimulus pair: F(6,168)=3.54; P = 0.003). The inter-action between processing condition and visual ®eldre¯ects the di�erence in RT to prime-target pairs pre-sented in the same and di�erent visual ®elds in the twoprocessing conditions. In the automatic condition, re-sponses to targets presented in a particular visual ®eldwere faster when preceded by a prime in the samevisual ®eld, than when the prime appeared in the con-tralateral visual ®eld (Fig. 1). In contrast, in the con-trolled condition, responses to targets appearing in agiven visual ®eld were faster when the prime was pre-sented in the contralateral visual ®eld (Fig. 2). Thethree-way interaction between these variables was alsomarginally signi®cant (F(6,168)=2.07; P = 0.059).Consequently, RTs for the automatic and controlledprocessing conditions were subsequently analyzed intwo separate ANOVAs (3 � 4) with type of stimuluspair and visual ®eld as the variables.

In the ANOVA computed for the automatic proces-sing condition, all e�ects were signi®cant. The maine�ect for stimulus pair (F(2,56)=17.14; P < 0.001)re¯ected an RT advantage for the related condition (at539 ms) relative to the neutral (561ms) and unrelatedconditions (554 ms). Thus, responses to related targetswere facilitated, while responses to unrelated targetswere slightly faster than the neutral baseline. The maine�ect for visual ®eld (F(3,84)=18.79; P < 0.001)re¯ected the RT advantage when targets wereprojected to the RVF (Fig. 1, Table 2). There wasalso an interaction between these two variables(F(6,168)=2.31; P = 0.036). To identify the source ofthis interaction, separate one-way ANOVAs were cal-culated for each visual ®eld condition, with type ofstimulus pair as the variable. The latter variable wascomprised of three levels: related, neutral and unre-lated word pairs. Two of these ANOVAs returned sig-ni®cant e�ects: when both prime and target wereprojected to the RVF (F(2,56)=9.668; P < 0.001) andwhen a LVF prime was followed by a RVF target

(F(2,56)=22.44; P < 0.001). For the remaining twovisual ®eld conditions, there was no di�erence betweenRTs for related, neutral and unrelated word pairs(LVF±LVF: F(2,56)=0.519; P = 0.598; RVF±LVF:F(2,56)=2.659; P = 0.079). Multiple pairwise com-parisons were used to ascertain whether the signi®cante�ects in the RVF±RVF and LVF±RVF conditionswere due to a facilitation of related targets or an inhi-bition of unrelated targets relative to their neutralbaseline. These comparisons were undertaken usingone-way ANOVAs, with Type I error controlled byadjusting the critical F to 6.125 using Tukey's test. ForRVF±RVF presentations, responses to related pairswere signi®cantly faster than responses in the neutralcondition (F(1,28)=12.44; P = 0.001), but responsesto unrelated pairs did not di�er from the baseline(F(1,28)=3.11; p = 0.089). Likewise, for LVF±RVFpresentations the facilitation for related pairs was sig-ni®cant (F(1,28)=36.74; P < 0.001) but there was noinhibition in response to unrelated pairs(F(1,28)=2.71; P = 0.111). Thus, when targets wereprojected to the RVF, the response pattern was one offacilitation for related targets, without inhibition forunrelated targets.

Of primary interest in these results is the asymmetricpriming of semantic category exemplars in each hemi-sphere. Responses to targets projected to the RVFwere facilitated if preceded by nonassociated categoryexemplar primes in either the LVF or RVF (Table 1).No such facilitation was present for targets projectedto the LVF. This indicates that in the automatic pro-cessing condition, semantic category meanings wereactivated in the left, but not the right hemisphere.Furthermore, there was no signi®cant di�erencebetween RTs for targets preceded by neutral or unre-lated primes in any visual ®eld condition, indicatingthat there was no inhibition in response to unrelatedtargets.

The ANOVA (3 � 4) computed for the controlledprocessing condition also returned signi®cant e�ects onall factors. The main e�ect for visual ®eld

Table 1

Facilitation and inhibition e�ects under automatic and controlled processing conditions

Automatic processing condition Controlled processing condition

Facilitationa Inhibitionb Facilitationa Inhibitionb

Visual ®eld

RVF±RVF +30c +10 +13 ÿ12LVF±RVF +37c +10 +11 ÿ26cLVF±LVF +7 +7 +23c +14

RVF±LVF +17 +1 +18c ÿ16a Facilitation=RT neutralÿRT related, where a positive value denotes facilitation.b Inhibition=RT neutralÿRT unrelated, where a negative value denotes inhibition.c Signi®cant comparison according to Tukey's test.

M. Collins / Neuropsychologia 37 (1999) 1071±10851078

(F(3,84)=8.52; P < 0.001) again re¯ected the RT ad-vantage when targets were projected to the RVF (Fig.2, Table 2). Like the automatic condition, the maine�ect for stimulus pair (F(2,56)=17.51; P < 0.001)re¯ected the RT advantage for related pairs (at 541ms) relative to the other two word conditions.However, unlike the automatic condition, RTs to unre-lated pairs were 10 ms slower than RTs to neutralpairs (at 567 ms and 557 ms respectively). This is con-sistent with ®ndings of a facilitation in response to re-lated targets in conjunction with slower responses tounrelated targets under controlled processing con-ditions [e.g. 9,25]. The interaction between visual ®eldand type of stimulus pair (F(6,168)=3.12; P = 0.006)was analyzed using separate one way ANOVAs foreach visual ®eld condition, with type of stimulus pairas the variable. All four ANOVAs returned signi-®cant e�ects (RVF±RVF: F(2,56)=4.911; P = 0.011;LVF±RVF: F(2,56)=14.584; P < 0.001; LVF±LVF:F(2,56)=4.282; P = 0.019; RVF±LVF: F(2,56)=9.883;P < 0.001). To ascertain whether these signi®cante�ects were due to facilitation or inhibition of re-sponses relative to the neutral baseline, multiple pair-wise comparisons were computed using a one-wayANOVA for each visual ®eld condition, with the criti-cal F adjusted to 6.125 via Tukey's test. It is importantto note that responses to related targets were fasterthan responses to targets preceded by neutral primesin all visual ®eld conditions, while responses to unre-lated targets were slower than the neutral baseline inall but the LVF±LVF condition (Fig. 2). In the lattercondition, RTs to unrelated targets were 14 ms fasterthan responses to the neutral baseline, although thisdi�erence was not signi®cant (F(1,28)=3.43;P = 0.075). This suggests that controlled processingwas evoked in all visual ®eld conditions, except theLVF±LVF condition. Even so, although inhibitionranged from 12 ms to 26 ms in the other three visual®eld conditions (Table 1), inhibition reached signi®-cance for LVF±RVF presentations (F(1,28)=10.29;P = 0.003), but not for RVF±RVF (F(1,28)=2.12;

P = 0.157) or RVF±LVF presentations (F(1,28)=3.32;P = 0.079). Facilitation was signi®cant in two visual®eld conditions: when both prime and target were pro-jected to the LVF (F(1,28)=9.84; P = 0.004) andwhen a prime in the RVF was followed by a target inthe LVF (F(1,28)=6.79; P = 0.014). There was no fa-cilitation for LVF±RVF (F(1,28)=3.57; P = 0.069) orRVF±RVF presentations (F(1,28)=3.99; P = 0.056).Further analysis indicated that the signi®cant e�ect inthe one-way ANOVA computed for the RVF-RVFcondition was entirely due to a di�erence between RTsfor the related and unrelated conditions(F(1,28)=7.73; P = 0.01), as there was no signi®cantfacilitation or inhibition in this condition.

Hence, for the controlled processing condition, eventhough inhibition reached signi®cance only when theprime was directed to the LVF and the target to theRVF, responses to unrelated targets were slower rela-tive to the neutral baseline in all visual ®eld conditionsexcept LVF±LVF presentations (Fig. 2). This suggeststhat controlled processes in¯uenced responses to someextent when either the prime or target was projected tothe left hemisphere. In contrast, for LVF-LVF presen-tations RTs to unrelated pairs were 14 ms faster thanto neutral pairs, although this di�erence did not reachsigni®cance. This is similar to the pattern of responsesfor word pairs projected to the right hemisphere in theautomatic processing condition. Thus, strategic pro-cesses may have been elicited in the left, but not theright hemisphere in this condition. In addition, re-sponses to LVF targets were facilitated if preceded bynonassociated category exemplar primes in either theLVF or RVF. No facilitation was present when thesetargets were projected to the RVF. So, in this exper-imental condition, only the right hemisphere wasprimed for semantic category relationships (Table 1).

3.2. Error Analyses

Error rates for the positive set were also analysed bymeans of a four-way ANOVA (2 � 2 � 3 � 4) on test-

Table 2

Standard Deviations associated with RT means in the automatic and controlled processing conditions

Visual ®eld of presentation

RVF±RVF LVF±RVF RVF±LVF LVF±LVF

Automatic processing

Related pairs 78.5 65.6 81.1 83.1

Neutral pairs 80.4 65.5 81.4 84.5

Unrelated pairs 78 68.9 84.2 83.6

Controlled Processing

Related pairs 86.6 70 76.6 84.1

Neutral pairs 74.9 72.9 79.5 84.1

Unrelated pairs 76.9 84.5 93.8 82.2

M. Collins / Neuropsychologia 37 (1999) 1071±1085 1079

ing session, processing condition, type of stimulus pairand visual ®eld. Generally, the outcomes were consist-ent with the RT analyses. Like the RT data, the maine�ect for session (F(1,28)=7.33; P = 0.011) re¯ectedsuperior performance in the second testing session ofboth the automatic and controlled processing con-ditions. However, unlike the RT data, session inter-acted with processing condition (F(1,28)=5.51;P = 0.026), where the improvement in accuracy notedin the second session of the automatic condition wasgreater than the improvement in the second session ofthe controlled processing condition. The main e�ectfor processing condition (F(1,28)=26.34; P < 0.001)re¯ected greater accuracy for the controlled than theautomatic processing condition (by 4.12%). This is notsurprising, as the temporal interval between prime andtarget was 500 ms longer in this condition, giving sub-jects more time to consider their response. Thus, unlikethe RT data, practice di�erentially in¯uenced re-sponses in the two processing conditions. Even so, ses-sion did not interact with type of stimulus pair orvisual ®eld, and did not enter into any higher order in-teractions. Thus, practice did not di�erentially in¯u-ence responses to stimuli projected to either visual®eld, nor responses to targets preceded by the varioustypes of word prime. This provides further evidencethat practice has not compromised these data. Theremaining two main e�ects were also signi®cant. Themain e�ect for visual ®eld (F(3,84)=17.16; P < 0.001)re¯ected greater accuracy when targets were projectedto the RVF (Table 3). Thus, responses were both fasterand more accurate when the target was projected tothe left hemisphere. For stimulus pair, the main e�ect(F(2,56)=15.4; P < 0.001) re¯ected greater accuracyfor targets preceded by a related prime (with 10.18%errors) relative to unrelated and neutral primes, whereaccuracy was almost equivalent (with 13.18% and13.56% errors respectively).

Like the RT data, there was also a three-way inter-action between processing condition, type of stimuluspair, and visual ®eld (F(6,168)=2.37; P = 0.032). Thisinteraction was analyzed with two separate ANOVAs(3 � 4) for the automatic and controlled processingconditions, with stimulus pair and visual ®eld as vari-ables. Session was not included as a variable in theseanalyses, as the initial ANOVA indicated that it didnot interact with stimulus pair or visual ®eld. For theautomatic condition, both main e�ects were signi®cant,but there was no interaction. The main e�ect forstimulus pair (F(2,56)=16.43; P < 0.001) was due tohigher accuracy in responding to targets preceded byrelated primes (with 11.48% errors) relative to neutraland unrelated primes, where accuracy di�ered by only0.4% (with 16.02% and 15.62% errors respectively).The main e�ect for visual ®eld (F(3,84)=10.35;P < 0.001) re¯ected the greater accuracy in responseto RVF targets (Table 3). The absence of an inter-action between visual ®eld and stimulus pair(F(6,168)=1.08; P = 0.374) indicated that the advan-tage for related targets was consistent across all visual®eld conditions. Similarly, for the controlled proces-sing condition, there was a main e�ect for stimuluspair (F(2,56)=5.08; P = 0.009) and visual ®eld(F(3,84)=14.66; P < 0.001), but no interaction(F(6,168)=1.79; P = 0.103). In this condition, re-sponses were most accurate when the target was pre-ceded by a related prime (with 8.89% errors) relativeto neutral and unrelated primes (with 10.34% and11.51% errors respectively). In relation to visual ®eld,accuracy was highest when a LVF prime was followedby a RVF target. This di�ers from the automatic pro-cessing condition, where responses were most accuratewhen both prime and target were projected to theRVF. Nevertheless, this parallels the RT data, wherein the controlled processing condition responses werefastest for LVF±RVF presentations, while in the auto-

Table 3

Percentage error for word pairs presented in each visual ®eld under automatic and controlled processing conditions

Visual Field of Presentation

RVF±RVF LVF±RVF RVF±LVF LVF±LVF

Automatic processing condition

Stimulus pair relationship

Related 9.48 10.52 12.83 13.1

Neutral 11.59 15.03 17.97 19.5

Unrelated 12.66 12.98 18.72 18.14

Mean 11.24 12.84 16.51 16.91

Controlled processing condition

Stimulus pair relationship

Related 8.1 4.97 9.64 12.84

Neutral 10.52 3.97 12.41 14.48

Unrelated 8.45 8.97 12.41 16.21

Mean 9.02 5.97 11.49 14.51

M. Collins / Neuropsychologia 37 (1999) 1071±10851080

matic condition responses were fastest for RVF±RVFpresentations (Figs. 1, 2). Accuracy for the neutral andunrelated word pair conditions also di�ered for theautomatic and controlled processing conditions. In theformer, responses to neutral pairs were somewhatmore accurate than responses to unrelated pairs forRVF±RVF and RVF±LVF presentations, while in thecontrolled processing condition, responses to neutralpairs were more accurate than responses to unrelatedpairs for all visual ®eld conditions except RVF±RVFpresentations.

Hence, the improvement in accuracy in the secondtesting session of the automatic processing conditionexceeded the improvement in the second session of thecontrolled processing condition. However, this inter-action between session and processing condition doesnot compromise the current results, as session did notinteract with either visual ®eld or type of stimuluspair, and the error data serves simply as a secondarydependent variable, to check for speed-accuracy trade-o�s. None were evident. Furthermore, session merelyexerted a general e�ect on the RT data, and did notinteract with any other variable. Hence, practice didnot confound the RT data in each condition, so thepriming e�ects in each hemisphere under automaticand controlled processing conditions can be interpretedunambiguously.

4. Discussion

The pattern of priming facilitation obtained in ses-sion 1 of the current experiment was the converse ofthe facilitation found in session 2. In the former, aprime presented in either the RVF or LVF facilitatedresponses to non-associated categorically related tar-gets subsequently presented in the RVF. No facili-tation was apparent when these targets were presentedin the LVF (Table 1). Evidently in the ®rst session, aprime directed to either hemisphere activated categoryexemplars in the left, but not the right hemisphere.However, in the second session, primes presented ineither visual ®eld facilitated responses to categoricallyrelated targets presented in the LVF, while no facili-tation was present when these targets were projectedto the RVF (Table 1). So, in session 2, priming ofsemantic category exemplars was restricted to the righthemisphere.

Before interpreting these facilitation e�ects, it isnecessary to determine whether automatic and con-trolled processes were measured as intended. In the®rst session, the pattern of facilitation without inhi-bition in all four visual ®eld conditions suggests thatautomatic processes were measured in both hemi-spheres, as planned (Table 1). Therefore the primingfacilitation, which was restricted to the left hemisphere

in this session, re¯ects automatic processes. Then, toinvoke controlled processes in the second session,stimulus pairs were presented in a manner designed toencourage subjects to focus their attention upon themeaning of the prime and thereby develop expectanciesabout likely targets. In this session, when either aprime or target was projected to the RVF, responsesto targets preceded by an unrelated prime were slowerthan responses to targets preceded by a neutral prime.This inhibition reached signi®cance for LVF±RVF pre-sentations, where responses to unrelated targets were26 ms slower than responses to the neutral baseline.There was also a trend towards inhibition for RVF±LVF and RVF±RVF presentations, where responses tounrelated targets were 16 ms and 12 ms slower thanthe neutral baseline respectively (Table 1). The slowerresponses to unrelated targets in the latter two con-ditions suggest that strategic processes were engaged inthose conditions, just as they were in the LVF±RVFcondition. This conclusion is bolstered by the obser-vation that in all conditions where either the prime ortarget was projected to the left hemisphere, responsesdi�ered substantially from the pattern of responsespreviously obtained in the automatic processing con-dition: while responses to unrelated targets wereslightly faster than their baseline in the ®rst session,they were slower than the baseline in the second (com-pare Figs. 1 and 2). Consideration of these factorstogether suggests that in session 2, strategic processesin¯uenced responses when the left hemisphere wasinvolved in processing either the prime or target,whereby the subject's attention was directed to relatedtargets after viewing a prime, so responses to unrelatedtargets were inhibited.

In contrast, when the prime and target were bothpresented in the LVF in session 2, there was no indi-cation of an inhibition in response to unrelated targets.Rather, like the automatic condition, responses tounrelated targets were slightly faster than responses totargets preceded by neutral primes (Fig. 2). Thissuggests that the facilitation e�ects observed whenprime and target were projected to the right hemi-sphere re¯ect automatic processes, even though the ex-perimental conditions were designed to optimize thelikelihood of engaging strategic processes. However,before concluding that the current experiment failedentirely to invoke strategic processes in the right hemi-sphere, di�erences in performance for within- relativeto cross-hemisphere presentations in the automatic andcontrolled processing conditions must be considered.

In the ®rst session, responses for a given hemispherewere faster and more accurate when prime-target pairswere presented in the same visual ®eld than when theywere presented in di�erent visual ®elds (Fig. 1).However, in session 2, responses were faster and moreaccurate when the target was preceded by a prime in

M. Collins / Neuropsychologia 37 (1999) 1071±1085 1081

the opposite visual ®eld (Fig. 2)3. It appears that theprojection of a prime word to a particular hemisphereacted as a general stimulant to the lexicon in thathemisphere, resulting in facilitated processing of allwords projected to the same hemisphere shortly there-after. Consequently, all target words projected to the`stimulated' hemisphere 100 ms after the prime, wereprocessed more quickly than targets projected to the`non-stimulated' contralateral hemisphere. In session 1,this resulted in better performance for within- thancross-hemisphere presentations of all words, irrespec-tive of their relationship to the prime. It must beborne in mind here, that the priming facilitation for re-lated targets in the RVF occurred over and above thisgeneral facilitation for all words. It is curious there-fore, that despite the apparent general stimulation ofthe right hemisphere lexicon by a LVF prime, therewas no additional priming facilitation for categoricallyrelated targets. This is unlikely to stem from the righthemisphere's lack of responsiveness to categorical re-lationships, as facilitation was evident for LVF presen-tations in session 2. It is possible, therefore, thatdi�use activation of the right hemisphere lexiconoccurs prior to any additional activation of categori-cally related words.

In session 2, responses for cross-hemisphere presen-tations were superior to those for within-hemispherepresentations. The absence of a general advantage forword pairs presented to the same hemisphere in thissession may partially be due to the initial general acti-vation of the lexicon having abated during theextended inter-stimulus interval, particularly as stra-tegic processes began to in¯uence processing.Furthermore, this interval provided subjects withgreater opportunity to consciously recognize bothprime and target words. In accordance with theinstructions for the controlled processing condition, itis likely that they used this as an opportunity to checkfor a relationship between the prime and target. In sodoing, subjects then had to decide whether to respondto the prime or target word, thereby complicating re-sponse preparation and delaying responses. Supportfor this suggestion is found in the 20 ms delay in RTsfor within-hemisphere presentations in session 2 rela-tive to session 1. Why then, was there no equivalentresponse delay for cross-hemisphere presentations? It isconceivable that response preparation was easier whenstimulus pairs were projected to di�erent hemispheres,as the prime was coded with additional informationdistinguishing it from the target, in the form of a `tag'indicating that the word had been processed in a

di�erent hemisphere [see 15,23]. This may haveresulted in superior performance for word pairs pro-jected to di�erent hemispheres relative to pairs pro-jected to the same hemisphere.

It is interesting that despite the response delay forword pairs projected to the same hemisphere in session2, priming facilitation was present for LVF±LVF pre-sentations. If conscious recognition of the prime andtarget delayed responses in this session, as proposedabove, then this facilitation may re¯ect the in¯uence ofstrategic processes operating in the right hemisphere togenerate semantic category aspects of word meanings.However, the absence of any accompanying inhibitionwhen unrelated targets were presented in this con-dition, and the similarity in pro®le of responses forLVF±LVF presentations in sessions 1 and 2, intimatesthat this facilitation re¯ects automatic processes.Failure to elicit inhibition in the right hemisphere isnot unusual [8,9]. Furthermore, quite di�erent pro®lesof categorical priming in each hemisphere have beenfound when primes are centrally or laterally presented[8]. It seems that the signi®cant data limitations forlateralized stimuli [29], particularly when presented ina lexical decision task with very brief exposure dur-ations [19], have a greater impact upon the processingof stimuli presented in the LVF than in the RVF.Con®rmation of this is seen in the higher error ratesand slower responses for LVF than RVF stimuli in thepresent study and previous research [e.g. 14,15,22].Evidence that the strategic processes underlying retro-spective semantic matching require a clearly perceivedprime [8,25], as is the case with central, but not lateralpresentations, suggests that a probable consequence oflateral prime presentation is an interference in theengagement of controlled processes in the right hemi-sphere. It is also possible that the right hemispherecontributes to linguistic processing primarily at anautomatic level of processing, and has access only toparticular strategic processes. If so, it is conceivablethat strategic processes in¯uenced processing of certainaspects of the stimuli in the current experiment, butwere not the source of the facilitation e�ects for LVF±LVF presentations. Consistent with this, Chiarello,Richards and Pollock [8] suggest that the right hemi-sphere can generate semantic expectancies, but doesnot utilize retrospective matching.

Hence, in the current experiment, non-associatedcategory exemplars were activated in the left hemi-sphere under automatic, but not controlled processingconditions. In the latter condition, priming facilitationwas restricted to the right hemisphere. However, thepro®le of responses indicate that this facilitationre¯ected automatic, and not strategic processes. If so,the facilitation e�ects in the current study are all at-tributable to automatic processes. This in turn suggeststhat both hemispheres have automatic access to

3 I would like to extend my thanks to an anonymous reviewer who

highlighted the importance of this aspect of the data, and suggested

an interpretation which I have expanded upon here.

M. Collins / Neuropsychologia 37 (1999) 1071±10851082

semantic category representations. However, since fa-cilitation was restricted to the left hemisphere in ses-sion 1, and to the right hemisphere in session 2, itmust be concluded that categorical representations areautomatically activated in both hemispheres, but underdi�erent circumstances. Furthermore, the present studyfound no evidence that the left hemisphere invokesconscious strategies to generate categorical aspects ofword meanings. Facilitation restricted to the left hemi-sphere in the automatic condition, in conjunction withthe absence of facilitation in this hemisphere in thecontrolled processing condition, provides clear evi-dence against this.

These results clearly indicate that one aspect of theleft hemisphere's on-going linguistic processing entailsautomatic access to semantic category representationsin memory. This is contrary the conclusion ofChiarello and her colleagues [6,7] that category mem-bers are more readily activated in the right than theleft hemisphere, with automatic access to categoricalrepresentations primarily a function of the right hemi-sphere. Moreover, since many of the exemplarsemployed in the current study were low in typicality,there is no support for the proposal that any acti-vation of categorical meanings in the left hemisphere iscon®ned to highly typical exemplars [7] with activationspreading in a more constrained manner in the leftthan the right hemisphere [7]. Rather, these results in-dicate that our previous ®nding of semantic categorypriming in the left hemisphere [14] re¯ects automaticrather than controlled processes. If so, the discrepan-cies between our research and that of Chiarello andcolleagues [6,7] cannot simply be attributed to themeasurement of controlled and automatic processingrespectively.

It appears that these discrepancies are attributableto di�erences in the timecourse of activation of seman-tic category meanings in each hemisphere. In the cur-rent study, priming was restricted to the lefthemisphere when stimulus pairs were separated by anSOA of 250 ms, and to the right hemisphere whenpairs were separated by 750 ms. Semantic categorypriming restricted to the left hemisphere has also beenobserved when the temporal interval between primeand target ranges between 165 ms and 450 ms [14,20]while category priming is restricted to the right hemi-sphere when these intervals exceed 574 ms [6,7,20].These ®ndings are in complete accord with the currentstudy, and indicate that the pattern of semantic cat-egory priming in each hemisphere varies with SOA.This is consistent with evidence that time is a majormediator of priming e�ects in each hemisphere, wherethe time course of activation di�ers in each hemi-sphere, with associates [13] and category exemplars[20] taking longer to activate in the right hemisphere.This is con®rmed by a comparison of the results of the

current experiment with those of Koivisto [20]. Bothstudies found category priming restricted to the lefthemisphere when the proportion of related pairs waslow and SOA short. Furthermore, both studies foundcategory priming restricted to the right hemispherewhen pairs were separated by an interval of 750 ms.This is despite the adoption of a low relatedness pro-portion in Koivisto's study and a high proportion inthe current experiment. It seems that relatedness pro-portion plays a minor role in in¯uencing the occur-rence of semantic category priming in eachhemisphere. Perusal of current evidence con®rms this:category priming restricted to the right hemisphere hasbeen found with stimulus sets comprised of relatednessproportions ranging from a low 19% [6], 24% [7] and25% [20] to a high of 50% in session 2 of the currentstudy. Likewise, while category priming restricted tothe left hemisphere has been found with a low pro-portion of related pairs (20% in session 1 of the cur-rent study; and 25% [in 20]) it has also been foundwith a high proportion of 50% [14]. The clearest dem-onstration of this point is seen by contrasting the ®nd-ings of the current study with those of Chiarello et al.[6]. To measure automatic processes, the two studiesused a relatedness proportion which di�ered by only1%, yet Chiarello et al. found category priming onlyin the right hemisphere, while the current study foundpriming restricted to the left hemisphere. It is interest-ing in this context, that the SOA used in the two stu-dies di�ered by 325 ms.

Therefore, the discrepancies between our research[12,14] and that of Chiarello and colleagues [6,7]appear to be primarily attributable to di�erences in theSOA employed. Although both sets of researchersappear to have measured automatic processes, thewidely divergent SOAs employed by them producedquite di�erent priming e�ects, because semantic cat-egory meanings take longer to be activated in the rightthan the left hemisphere. This conclusion is con®rmedby comparing the results of (Collins) Abernethy andConey [14] with those obtained in session 2 of the cur-rent study: even though the same proportion of categ-orically related pairs was used in both studies (i.e.50%), priming was restricted to the right hemispherein the present study, and to the left hemisphere in ourprevious study [14]. The important di�erence betweenthese two studies is the SOA employed. In our earlierstudy, stimulus pairs were temporally separated by 250and 450 ms, while in this study they were separated by750 ms. This con®rms that SOA is more importantthan relatedness proportion in determining the pro®leof semantic category priming in each hemisphere.

When taken in conjunction with similar research,these ®ndings are useful to derive estimates of thetimecourse of activation of category representations ineach hemisphere. Current evidence suggests that

M. Collins / Neuropsychologia 37 (1999) 1071±1085 1083

semantic category representations are automaticallyactivated in the left hemisphere within 165 ms of thepresentation of a prime [20] and remain active for 450ms [12,14 and the current study]. The absence of cat-egory priming in the left hemisphere after 450 ms[6,7,20 and the current study] suggests that the acti-vation of categorical meanings which stems from auto-matic processes, dissipates in the left hemispherewithin 500 ms. It is helpful to consider these ®ndingsin light of Burgess and Simpson's proposal [4] that lefthemisphere semantic processing operates in a focalmanner, with rapid selection of one meaning and sup-pression of other potential candidates. Conceivably,the ®rst 500 ms of linguistic processing in the lefthemisphere primarily involves access to a broad rangeof meanings through the automatic spread of acti-vation. As part of this process, category meanings[12,14,20 and the current study], as well as associates[13] and alternate meanings of ambiguous words [4]are quickly accessed in the left hemisphere. Then,around 500 ms after the presentation of a prime, con-trolled processes begin to in¯uence on-going linguisticprocessing in the left hemisphere, so the focus of atten-tion turns to a small set of closely related meaningswhile less likely meanings are suppressed. Current evi-dence indicates that semantic category meanings areamongst those suppressed in the left hemisphere at thisstage [6,7,20 and the current study], with attention di-rected to associates [9] and dominant meanings of theprime [4,16]. The suggestion that 500 ms is the timewhen controlled processes begin to manifest in the lefthemisphere is bolstered by evidence that performancere¯ects automatic processes for the ®rst 400±500 msafter the presentation of a prime, and begins to re¯ectstrategic processes after 500 ms [24].

In the right hemisphere, on the other hand, the time-course of activation is retarded relative to the lefthemisphere [13,20], with semantic category meaningstaking around 575 ms to be primed therein [6,20] butremaining active for at least 750 ms [7,20 and the cur-rent study]. It is interesting in this context, that cat-egory meanings are activated in the left hemispherewithin 165 ms but suppressed by 500 ms, and it isaround 500 ms that category priming begins to mani-fest in the right hemisphere. This raises the possibilitythat semantic category meanings become available inthe right hemisphere once they have been suppressedin the left. Again, this is consistent with Burgess andSimpson's proposal [4] that the right hemisphere's dif-fuse semantic processing style maintains a wide rangeof semantic meanings over an extended time, so thatthey remain available for processing if required at alater stage to disambiguate text. In light of this, itseems reasonable to propose that the retardation of ac-tivation (and possibly inhibition) of semantic meaningsin the right hemisphere is one mechanism by which a

wide range of related meanings are available at a stagewhen inhibitory processes have reduced the number ofactive representations in the left hemisphere to a smallsubset of closely related meanings [4,11,18].

The results of the current study are also consistentwith evidence that word meanings are automaticallyactivated in both hemispheres, but the inhibition ofunrelated words occurs only in the left hemisphere[8,9]. In session 2, `word' responses for unrelated pairswere stalled relative to responses for neutral pairs, butonly when the left hemisphere was involved in proces-sing either the prime or target. This was not the casewhen both the prime and target were projected to theright hemisphere. These ®ndings are consistent withthe view that the left hemisphere alone uses a retro-spective matching strategy [8] where the relatednessbetween a prime and target is used after lexical accessto decide whether the target is a word. Even so, inhi-bition in the left hemisphere is usually accompanied byfacilitation of related pairs [8,9] which has promptedthe suggestion that the left hemisphere plays a primaryrole in postlexical semantic integration [9]. This wasnot the case in the current study, where there was nopriming facilitation in the left hemisphere, even thoughresponses to unrelated targets were inhibited therein.One explanation for the absence of left hemisphere fa-cilitation in the controlled processing condition of thecurrent study is that attention was directed to closelyrelated meanings, but not to more remotely relatedmeanings like category exemplars. Afterall, those stu-dies which have found facilitation in the left hemi-sphere under controlled processing conditions haveused stimulus pairs which were more closely relatedthan the exemplar pairs employed in the current study(e.g. associates [9] and category exemplars which werealso associated [5,8]). In addition to this, evidence thatthe processes which underlie strategic retrospectivematching require a clearly perceived prime [8], andthat controlled processes are engaged with central, butnot lateral presentations of a prime [8], raises thepossibility that the conditions of the current studywere inappropriate for strategic processes to manifestan in¯uence on the processing of category exemplars.

In conclusion, there is no support in the currentstudy for the contention that the right hemisphere con-tributes to the activation of semantic category rep-resentations at an automatic level of processing whilethe left hemisphere invokes conscious processes to gen-erate categorical information. Rather, the left hemi-sphere has automatic access categorical aspects ofword meaning and these meanings are available earlierin the processing sequence than in the right hemi-sphere. Although the current study found evidencethat the right hemisphere also has access to categoricalaspects of word meanings, it is not as clear whetherthis involves automatic or strategic access to these

M. Collins / Neuropsychologia 37 (1999) 1071±10851084

memory representations. These results also providesome support for the suggestion [9] that the left hemi-sphere plays a primary role in strategic processing.Furthermore, consistent with Burgess and Simpson'sproposal [4], it appears that semantic category mean-ings may become available in the right hemisphereafter strategic processes have rendered them unavail-able to the left hemisphere. This study provides furtherevidence that both hemispheres make a signi®cant,albeit di�erent, contribution to linguistic processing.

Acknowledgements

This research was supported by an AustralianResearch Council grant (File number: 0320198193). Ithank Dr Je�rey Coney for the use of his software, MsKylie Marsh for collecting the data, and two anon-ymous reviewers for their helpful comments.

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